FIGS. 2 to 6 of Japanese Patent Document JP-A-2000-315563 discloses a method for manufacturing an insulator for a spark plug. This manufacturing method forms a through hole in the insulator for inserting a center electrode and a terminal electrode in an axial direction.
In this manufacturing method, first, as a preparing step, a press pin used to form the through hole is prepared and a forming die is prepared that has a cavity in which an opening is formed on the rear end side in the axial direction. The press pin is formed at a rear end thereof with a rib-shaped pin-side spiral portion which turns spirally around its own outer peripheral surface.
Next, as a powder filling step, green powder is loaded and filled into the cavity from the opening. Next, as a press pin arranging step after the powder filling step, the press pin is arranged within the cavity by moving the press pin toward a tip side of the forming die in the axial direction from the opening. As a cavity blocking step after the press pin arranging step, the opening is blocked by a blocking member. Then, as a compression molding step, the green powder within the cavity is pressed along with the press pin to obtain a compact (i.e., compact piece).
As a die releasing step after the compression molding step, the compact along with the press pin are removed from the cavity. As a press pin removing step after the die releasing step, the press pin is pulled out of the compact. The compact obtained in this way has an external shape corresponding to an insulator for a spark plug. Grinding is performed on this compact to form a green (non-sintered) insulator.
Then, the green insulator is sintered at a temperature of 1400° C. to 1650° C. Thereby, a pinhole formed by the press pin becomes the through hole. Then, the sintered insulator is coated with glaze and finish-sintered to provide an insulator for a spark plug. This insulator for a spark plug is assembled together with a center electrode, a terminal electrode, a metal shell, and a resistor, and becomes a spark plug. This spark plug is attached to an engine by a threaded portion of the metal shell, and is used as an igniting source for an air-fuel mixture supplied to a combustion chamber.
The diameter of the spark plug tends to decrease in order to save space, and thus the diameter of an insulator for the spark plug is correspondingly required to become smaller. For this reason, it is necessary to decrease the diameter of the through hole of the insulator for the spark plug. Thus, the insulator for the spark plug should be manufactured using the press pin whose diameter is small. However, in the above manufacturing method, when an insulator for a spark plug is manufactured using the press pin whose diameter is small, the press pin may be bent due to the resistance which the press pin receives from the green powder within the cavity in the press pin arranging step after the powder filling step. In this case, the pinhole of the compact does not extend straight in the axial direction, and thus the through hole of the insulator for the spark plug does not extend straight in the axial direction. For this reason, the insulator for the spark plug may become a defective article, or replacement of the press pin should be required frequently, and thus cause an increase in manufacturing costs.